Embodiments of the present invention are directed to delivering a cold gas at a controlled temperature to a vessel using a cryogen to maintain the temperature of the cold gas.
Many methods exist for supplying a cold gas at a controlled temperature to a vessel. Examples include mechanical cooling of a gas (compression & evaporation of a refrigerant), allowing a liquid cryogen to vaporize prior to being supplied to the vessel, and using a variable flow-rate “throttling gas” to control the temperature at which a cryogen is supplied to the vessel.
There are, however, several problems associated with these methods. Mechanical cooling requires use of refrigerants, such as fluorocarbons, ammonia, sulfur dioxide, and methane, which are toxic and/or environmentally hazardous. In addition, mechanical cooling is very inefficient at very low temperatures (e.g., below zero degrees C.).
Methods in which the cooling gas consists primarily of a vaporized liquid cryogen are susceptible to delivering at least some cryogen in liquid phase. Any surface in the vessel that comes in contact with the liquid phase cryogen is, therefore, subjected to intense, concentrated cooling. This is undesirable in applications in which the product being cooled in the vessel may be damaged by contact with the liquid phase cryogen and/or where the product is not intended to be frozen.
PCT International Application No. PCT/US08/74506, filed Aug. 27, 2008, discloses a cryogenic cooling system in which a cryogenic fluid is supplied at a constant flow rate and the flow rate of a “throttling gas” is used to control the temperature of a resultant fluid using temperature feedback from the resultant fluid flow stream. This type of system, however, exhibits poor performance characteristics if the coolant gas (resultant fluid) is supplied at relatively high flow rates, e.g., 3700 standard cubic feet per hour (SCFH) or higher, which are desirable for many applications. In addition, the temperature feedback sensor for this type of system must be placed in the resultant fluid supply line, preferably just downstream from the point at which the cryogenic fluid and throttling gas supply lines intersect. This is an undesirable limitation in applications in which it is desirable to have temperature feedback from the material being cooled or the vessel into which the resultant fluid is being discharged. Also, in order to provide stable resultant fluid temperature characteristics, the cryogenic fluid must be supplied using a specialized hose that minimizes vaporization of the cryogenic fluid, such as the triaxial cryogenic fluid supply line.
Accordingly, there is a need for an improved system and method capable of delivering a temperature-controlled cooling gas at relatively high flow rates, at a wide range of temperatures (including well-below zero degrees C.) and in a cost-effective manner. This need is addressed by the embodiments of the invention described herein and by the claims that follow.